The invention pertains to instruments used in the field of medicine and more particularly to an aspirator and method for taking a blood sample.
In taking a blood sample, it is customary to insert the cannula into a vein and withdraw a sample into a syringe. In the use of a conventional cannula it is necessary to accurately position the point in the vein in such a manner that the opening at the point is not blocked. For example, an 18-gauge cannula has a lumen of about 0.03 inch diameter. If the penetrating point is at a 30.degree. angle, the axial length of the end opening is only about 0.06 inch. This emphasizes the problem of accurately positioning the point. If the cannula is inserted too deeply, the point may pass through the vein and the opening will be blocked. If the opening is otherwise positioned adjacent the wall of the vein, it may be blocked, or it may become blocked when a slight vacuum is applied in drawing blood. It is desirable to have a device that alleviates the necessity of accurate positioning, without affecting the operability. Additionally, the material punctured by the cannula has a tendency to bulge into the lumen and cause what is commonly known as "coring" or "heel shaving". When this occurs, a piece of the material plugs the lumen so that it is difficult to draw a blood sample. It is desirable to have a device which alleviates the problem of "coring", without affecting the operability of the aspirator.
In the practice of pediatrics, it is frequently necessary to take a blood sample of an infant -- perhaps a new-born baby. Such a sample may be taken from the femoral vein and, while it is usually possible to pierce the vein, to accurately position the point of a 25-guage cannula is difficult at best. The femoral vein is near the femur and this fact is used in taking the sample. A cannula is inserted in the infant's thigh until it contacts the femur, the cannula is then slowly retracted until the open end communicates with the femoral vein. Obviously this is a difficult operation. It is desirable to provide a method which alleviates the necessity of accurately positioning the end of the cannula.